1. Field of the Invention
This invention relates to a transponder in a vehicle device on the vehicle side identification.
2. Description of the Related Art
As a conventional vehicle identification device, devices such as shown in Japanese Unexamined Utility Model Publication No. 57-159180 or Japanese Unexamined Patent Publication No. 61-201179 are well known.
In these conventional vehicle identification devices, a transponder, also called an identification tag (tag in short) modulates an unmodulated microwave transmitted from an interrogator with an ID code stored in a memory means provided in the tag and returns the modulated microwave back to the interrogator. Information inherent to a certain vehicle on which the tag is mounted is thus reported to the interrogator.
One example of the construction of a conventional vehicle identification device is shown in FIG. 1.
In FIG. 1, the conventional vehicle identification device is provided with an interrogator 1 and a tag 2. The interrogator 1 transmits an unmodulated microwave generated from a microwave oscillator 10 from an antenna 13 through a circulator 11 and demodulates the microwave received by the antenna 13 utilizing a demodulator 12 to output the demodulated microwave.
The tag 2 is designed so as to enable an electric power source 23 for a modulator 25 and a ROM 24 to be switched ON only when the unmodulated microwave transmitted from the interrogator 1 is received by the antenna 21 and detected by a detector 22, in order to prevent excess consumption of electric power of a battery.
Thus, when the electric power source 23 is ON, the tag modulates the microwave thus received in response to a coded signal stored in the ROM 24 and transmits back the modulated microwave to the interrogator 1.
In the conventional technology as shown in FIG. 1, the tag is generally required to have a detecting antenna 21, a detector 22, and an electric power source controlling circuit 23, in addition to a modulating antenna 26 and a modulator 25, to perform its inherent operation.
Among these components, the detecting antenna 21 modulating antenna 26 and the detector 22 and modulator 25 constitute distributed constant circuits. Therefore, it is difficult to minimize the size thereof to that of another lumped constant circuit.
Especially, the antennas 21 and 26 are required to have at least a predetermined area to obtain a gain high enough to operate the tag normally and thus the area occupied by these antennas is the largest among those of other components in the tag.
Accordingly, it is difficult to minimize the size of the tag remarkably in a conventional tag having two antennas.
The object of the present invention is to overcome these technological problems in the conventional tag and to provide a tag with a minimized size.